The invention relates to a brake control valve assembly which includes a pair of dual stage brake control valves.
A known dual stage brake control valve is described in U.S. Pat. No. 4,738,109, issued 19 Apr. 1988 and assigned to the assignee of this application. Such a brake valve provides the capability for manual braking when pump pressure is not available at the brake valve pump pressure inlet. This capability is achieved with a dual stage valve arrangement that includes a prefill piston and a high pressure piston. The prefill piston provides the large volume of oil and associated low hydraulic pressure required for engagement of the rear axle brake components. Once this volume is satisfied, the brake valve transitions from the larger prefill piston diameter to the smaller diameter high pressure piston in order to generate higher pressures required for braking.
With pump pressure available at the brake valve pump inlet, application of the brake valve (prefill piston/high pressure piston) requires only a small amount of travel from the neutral position to the pressure metering position. However, during this travel, fluid is substantially trapped and pressure is generated in the prefill chamber, resulting in a heavy pedal feel. The heavy pedal feel and associated higher pedal force required to apply the brake results in reduced control of the brake pressure due to the eventual pressure decay of the prefill chamber. This heavy pedal feel is more noticeable in cold weather conditions due to the increased pressure decay time.